The present invention relates to a thermosetting powder composition which can be used to prepare self-extinguishing coatings on objects, for example, electrical components, to protect them from the environment.
More particularly, the present invention is concerned with epoxy resin-based powder compositions which can be used to form protective coatings on electrical components, which coatings exhibit a self-extinguishing action in case of fire resulting from an external heat source or from overheating as a result of failure or breakdown of the electrical component.
In the present specification and claims, the term "self-extinguishing" means that the coating, if it catches fire, is capable of extinguishing itself, without any assistance.
Conventionally, self-extinguishing coating compositions contain epoxy resins having an epoxy equivalent of 450-1000, a fluidizing agent, a hardening agent (e.g. an acid anhydride), a catalyst (e.g. a tertiary amine) in an amount of 0.4-1.5% by weight based on the weight of the composition, a pigment, and a filler. These coating compositions are usually applied by a "fluidized bed" immersion technique, whereby the composition, in powder form, is brought into the state of a suspension by means of gases or air flowing upward through an appropriately vibrating porous baffle which supports the powder, thus producing a fluidized bed of the powder. The filler employed in these conventional coating compositions has the function of providing an appropriate particle size distribution for the composition which enables the fluidized bed to be formed during use of the composition in the fluidized bed immersion technique.
The conventional self-extinguishing coating compositions which are based on an epoxy resin are those in which the epoxy resin contains a halogen, such as bromine or chlorine, in a rather substantial amount. In fact, the self-extinguishing capability of such conventional epoxy resin-based coating compositions is achieved by using an epoxy resin containing a substantial amount of halogen. This creates a rather serious disadvantage in that, when the composition is heated, the halogen is liberated, giving off toxic fumes. Some such fumes are produced while the coating composition is being applied to the object to be coated. Additionally, of course, substantial fumes are given off when the coated object catches fire. In both cases, the net result is to create a danger to human health.
Concerning the manner in which the coating composition based on an epoxy resin has conventionally been applied to the object to be coated, this can be accomplished by heating the object to be coated to a temperature higher than the softening point of the coating composition, and immersing the object several times in a suspension of the powder (by the fluidized bed immersion technique discussed above) for a certain period of time sufficient for the powder to soften, adhere to and melt on the heated surfaces of the object to be coated, thus forming a homogenous and continuous protective coating on the object. The thus coated object is then heated a second time to cause gelling and subsequent polymerization in the composition to obtain a uniform, hard protective coating.
In addition to causing a health problem as mentioned above, some of the substances, such as chlorine, in the conventional epoxy resin-based self-extinguishing coating compositions exhibit a marked deleterious effect on the degree of pigmentation of the coating if the coated objects are exposed to ultraviolet rays.